Sounds like my question has the potential to be a heated debate. These are all good insights guys - much appreciate the time you took to reply.
It seems like all I would need the Genset for is the roof AC and possibly the Microwave. The cabin part of the Navion seems small enough to be cooled by the chassis while underway. While stopped, that's a different story. However, I think much of my camping will be done in Autumn so I'm not sure how much use the AC would be. It will certainly be nice to have over summer. I think I'll have to put some serious consideration to how much summer camping I foresee. As some of you have pointed out, its better to already be prepared for the unforeseen than to be stuck without.
*On a side note* As far as Microwave, it looks like this appliance uses about 1000W of power. Would I be able to use a beefed up house battery pack to power that? The Navion comes with two batteries. If I beefed it up to 4 and got an suitable inverter, would this suffice Microwave usage?